The aircraft was extensively modified, including BERP III type blades and 1,200 shp Rolls-Royce Gem 60 engines, amongst many other modifications. More details of the aerodynamic technology behind the record can be found in the AHS Vertical Flight Library, part of the AHS Online Store: The Aerodynamics of the Helicopter World Speed Record, F. J. Perry, Westland Helicopters Ltd., presented at Forum 43, May 1987.

What about the Russian PSV?

Interfax news stated that the Russian Helicopters Mi-24LL PSV demonstrator has flown to a cruise speed “greater than 405 km/hr (219 kt).” The news report was in late October 2016, though unconfirmed reports suggest that it had taken place several weeks earlier. Nonetheless, neither the company nor any Russian entity is claiming this as a record. To be a record to dethrone the G-LYNX, an aircraft would need to fly a sustained, average speed greater than 216 kt/400 km/hr over a similar distance, in opposite directions to account for wind differences.

The Mi-24LL is a single-seat "Flying Laboratory" ("LL" in Russian) modification of the Mi-24 Hind. The high-speed demonstrator uses blade tips that are similar in shaping to the BERP blades (sometimes jokingly refered to as "RERP" blades). More information on and references for this aircraft is found in the AHS Vertipedia entry.

What About Compound Helicopters?

The FAI has never recognized an official speed record for a wing-augmented and/or thrust augmented helicopter. Although Sikorsky stated that they had received approval from the National Aeronautic Association (NAA, the FAI representative organization in the United States) that the X2 would be considered a helicopter for purposes of a record because it did not have wings, Sikorsky management decided not to test the aircraft beyond its stated goal of 250 kt. As a result, the current helicopter record set by G-LYNX still stands today.

Many other compound helicopters have been built and tested. A few unofficial high-speed tests of note:

The Sikorsky X2 (with a pusher propeller and rigid coaxial counter-rotating rotors) reached a level flight speed of 252 kt, as well as 262 kt in a shallow 2° - 3° dive.

More than two dozen compound helicopters have flown since the 1930s, when the helicopter was first invented. What exactly is a "compound helicopter"? Well, according to Principles of Helicopter
Aerodynamics by Prof. J. Gordon Leishman (Cambridge University
Press, 2003):

A compound helicopter involves a lifting wing in addition
to the main rotor (lift compounding) or the addition of a separate
source of thrust for propulsion (thrust compounding).... The
idea is to enhance the basic performance metrics of the helicopter,
such as lift-to-drag ratio, propulsive efficiency, and maneuverability.
The general benefit can be an expansion of the flight
envelope compared to a conventional helicopter.

In other words, a “compound” may involve both lifting
and/or propulsive compounding. The goal is to “off-load” the
rotor from its normal lifting and propulsive duties. This can be
done by using a wing and/or auxiliary propulsion.

Putting the Record Straight (signed by author, David Gibbings). This 20 page booklet — written by the then-Deputy Chief Flight Test Engineer for Westland — details the efforts leading up to the record attempt.